Drop-On-Demand (DOD) liquid emission devices have been known as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators such as are disclosed by Kyser et al., in U.S. Pat. No. 3,946,398 and by Stemme in U.S. Pat. No. 3,747,120. A currently popular form of ink jet printing, thermal ink jet (or “thermal bubble jet”), uses electrically resistive heaters to generate vapor bubbles which cause drop emission, as is discussed by Hara et al., in U.S. Pat. No. 4,296,421. Although the majority of the market for drop ejection devices is for the printing of inks, other markets are emerging such as ejection of polymers, conductive inks, or drug delivery.
The printhead used for drop ejection in a thermal inkjet system includes a nozzle plate having an array of ink jet nozzles above ink chambers. At the bottom of an ink chamber, opposite the corresponding nozzle, is an electrically resistive heater. The ink chamber, nozzle plate, and heater are formed on a substrate, typically made of silicon, which also contains circuitry to drive the electrically resistive heaters. In response to an electrical pulse of sufficient energy, the heater causes vaporization of the ink, generating a bubble that rapidly expands and ejects an ink drop from the ink chamber. Ink is replenished to the ink chamber through ink feed channels, located adjacent the ink chamber, typically formed through the silicon substrate on which the ink chambers are formed.
The ink feed channels of the prior art have been formed in various ways using laser drilling, wet etching, or dry etching of the silicon. Printheads are typically fabricated using silicon wafers. The ink feed channels of the prior art has a long slot formed by patterning and etching through the silicon wafer from the back or non-device side. Most printheads of the prior art, use a single long slot for each color of ink. Multiple long slots are therefore formed in a thick silicon substrate, one for each color.
There is a desire to increase the number of nozzles on a printhead for each color. It is also desirable to decrease the spacing between ink feed channels to shrink the size of the printhead for lower cost. Increasing the number of nozzles increases the length of the printhead and therefore the length of the ink feed channels. This long channel in the silicon substrate will weaken the printhead making it more susceptible to stress cracking. Co-pending application (U.S. Publication No. 2008/0136867 A1), discloses the use of anisotropic dry silicon etch, utilizing the “Bosch” process (also known as pulsed or time-multiplexed etching), in which ribs are formed to break up the ink feed channel into sections to increase the strength of the printhead making it more extensible.
However, there is also a desire to increase the frequency of drop ejection. One limitation on the frequency of drop ejection is the time required to refill the ink chamber after the previous drop ejection. The frequency of drop ejection can be increased, if the time required to refill the ink chamber is decreased. Co-pending application (U.S. Publication No. 2008/0180485 A1), discloses a dual feed printhead in which the ink feed channel is replaced by multiple ink feed holes for each ink color, with the ink feed holes located on both sides of the ink chamber. In this case, long ink feed channels on both sides of the ink chamber cannot be utilized, as they would result in a considerable decreased strength for the structure.
In the dual feed printhead, therefore, the preferred ink feed openings are much smaller than the ink feed channels of the prior art, with lengths extending across 1-2 nozzles corresponding to a length of 20-100 μm and similar width. The use of these multiple feed holes, provide strength and extensibility to the printhead. However these small openings cause fabrication issues. Such small feature sizes cannot be formed using wet etching or laser etching. Instead, a dry anisotropic etch process utilizing the “Bosch” process must be used. For dry etching of small openings with high aspect ratio the etch rate is much slower than for large slots, and slows down further the deeper the etch proceeds, therefore increasing the etch time for formation of these holes. The silicon substrate can be thinned prior to etching to decrease this etch time. It is also desirable to thin the substrate to reduce viscous drag of ink through these small holes, so that ink refill time can be decreased. In fact, silicon substrate thicknesses less than 200 μm are desired to minimize the effect of viscous drag on the ink refill time, and to provide a good aspect ratio for high etch processing throughput during fabrication. However, processing of such thin wafers to pattern and etch the ink feed holes through the back of the wafer is difficult, resulting in wafer breakage and yield loss. It is, therefore, desirable to form ink feed holes along with minimizing the process steps on thin wafers.
Another method to decrease the viscous drag is by varying the ink feed opening versus the depth of the feed hole. In the prior art wet etching has been used to provide an anisotropic etch where the feed channel opening is wider at the back of the substrate and narrows down to a smaller opening at the front of the substrate next to the ink chamber. However, the sidewall angle for this, wet etch process of 54.74° is large, and for closely spaced ink feed channels, wet etching is not possible. The anisotropic dry silicon etch, utilizing the “Bosch” process produces openings that typically remain the same width or are reentrant in profile through the substrate in the opposite direction that is desired. It is, therefore, desirable to have a process where the ink feed opening is narrower at the front of the substrate adjacent the ink chamber and wider at the back of the substrate, but where the sidewall angle is significantly less than 54.74°.
In the dual feed printhead, to minimize the ink refill time, the ink openings are located very close to the ink chamber. Alignment of the ink feed openings to the ink chamber is critical. In prior art, the patterning of the ink feed channels is performed using back to front wafer alignment of a mask. However, there are issues in fabrication that degrade alignment. If the silicon wafer is warped the ink feed channels will not align precisely with the mask. Also, during the etch process itself, the etch direction is not completely perpendicular to the wafer surface, especially approaching the wafer edge, due to directional variation of the ions. It is also difficult to time the etch process so that there is no over etching causing undercut of the silicon wafer at the device side. It is desirable to have a process that self-aligns the ink feed channel to the ink chamber.
In forming the ink feed holes through the wafer from the back, the etching of the silicon stops on material used to form the ink chamber. The timing of the endpoint is critical as over etching causes undercut of the ink feed opening at the front surface that causes misalignment of the ink feed opening. Under etching of the area for the ink feed opening could yield a partially formed ink feed opening or even an entirely closed ink feed opening, which is undesirable. Since the etch rate is not uniform across the wafer there will always be ink feed openings that will be overetched. It is desirable to have a process that self aligns the ink feed opening to the ink chamber resulting in uniform ink feed openings with no undercut.
There is, therefore, a need for a printhead that has small ink feed holes aligned to the ink feed chambers that are easily fabricated with high yield. This printhead should also be capable of ejecting drops at high frequencies with an ink chamber refill capability to meet this ejection frequency requirement.